Modern civil aircraft typically cruise at between 30,000 and 40,000 feet, which places them above most weather systems, except for towering cumulonimbus clouds and their associated electrical storms. As these can present a hazard, most airliners are equipped with weather radar that can detect them and enable the pilot to take avoiding action. However, there remain a number of adverse atmospheric conditions that are desirable to detect for which suitable avionic systems are not presently available. These include volcanic ash, toxic gases such as sulphur dioxide gas, wind-blown dust and ice particles.
Volcanic clouds contain silicate ash and gases that are hazardous to aviation. Several encounters between jet aircraft and volcanic ash have resulted in significant damage due to ingestion of ash into the hot parts of the engine, subsequent melting and fusing onto the turbine blades. Ash can also block the pitot static tubes and affect sensitive aircraft instruments, as well as abrade the leading edges of parts of the airframe structure.
Volcanic gases, principally sulphur dioxide (SO2), whilst less dangerous to aircraft than volcanic ash, do pose a hazard in themselves. In addition, their presence can be used as an indicator of volcanic ash as these substances are often co-located and are transported together by atmospheric winds. SO2 clouds from volcanoes will react with water vapour in the atmosphere to produce sulphuric acid which can damage aircraft.
Another important gas in volcanic clouds is water vapour (H2O gas). Water vapour occurs in copious amounts in volcanic clouds, either through entrainment of ambient air or from water from the volcanic source (e.g. sea water is a common source for volcanoes on islands or in coastal regions). Once in the atmosphere, the water vapour can condense on ash which act as nuclei, rapidly forming ice with a much smaller radius than ice in normal meteorological clouds. These abundant, small-sized ice particles are hazardous to aircraft because the rapid melting of the ice when in contact with the hot engines releases the ash nuclei which then fuse onto the turbine blades, affecting the engine performance and potentially causing the engine to stop.
Damage to aircraft resulting from encounters with volcanic clouds can be counted in the millions of dollars. Most serious aircraft encounters with ash clouds have been at cruise altitudes, but there is also a hazard to aircraft at airports affected by volcanic ash. These airports are usually close to an active volcano but they can also be at some distance from the source of the eruption due to atmospheric transport that brings ash into the region.
The cost of ash hazards to airport operations is not known, but must be significant if the costs include those due to delays to landings and take-offs as well as re-routing costs incurred by airline operators. The April 2010 eruption of Eyjafjallajoekull in Iceland is estimated to have cost the airline industry approximately US$2 billion.
Although there are currently no regulatory requirements for airport operators to provide warnings of ash hazards, warnings are issued based on information from volcano observatories, meteorological advisories and, in some cases, radar observations of eruption columns. Radar information is generally only reliable at the start of an eruption when the ash cloud is thick and usually such information is only available at airports in close proximity to an erupting volcano. For airports distant from the source of ash there are few direct observations available. Some observations come from satellite systems and other sources of information come from trajectory forecasts based on wind data and cloud height information. Much of this information is sporadic and untimely and there is a need for better detection systems.
Whilst volcanic clouds are the best known example (apart from thunder clouds) of an adverse atmospheric condition, there are other such hazards. For example, other non-ash particles can also under the right conditions initiate ice particle formation when they form nuclei around which water freezes. Ice crystals may accrete within turbine engines and are believed to be the cause of a number of power loss events. In addition, there is the possibility of toxic or otherwise dangerous gasses being emitted from industrial sources.
Jet aircraft at cruise altitudes (i.e. above 15,000 feet), travel rapidly (>500 km hr−1) and currently do not have a means for detecting volcanic cloud hazards ahead. Because of the high speed, a suitable detection method must be able to gather information rapidly and provide an automated alert and be capable of distinguishing volcanic substances from other substances in the atmosphere (e.g. meteorological clouds of water and ice).
U.S. Pat. No. 5,654,700 proposes a volcanic cloud detection system which displays the position of a volcanic cloud relative to the aircraft's position and thereby enables the aircraft to route around the cloud. The system operates by comparing the absolute and relative values of brightness temperatures detected at a number of specific infra-red wavelengths to certain threshold values. However, there is no disclosure about how these threshold values are determined, except that they are calculated using a microprocessor and depend on the altitude and attitude of the aircraft.